1. Field of the Invention
This invention relates to molten metal coating of metal sheet in continuous strip form and more particularly to an improved bearing support system for the rolls in a molten metal bath of a hot dip continuous metal strip coating apparatus.
2. Description of the Prior Art
Although there have been numerous improvements in hot dip metal coating in recent years, these improvements generally have been in the area of control and the alloys used as the coating metal. Since hot dip zinc and zinc alloy coating (herein generally referred to as galvanizing) is the most common form of molten metal coating operation, the invention will be described with reference to a galvanizing operation, it being understood that the invention may also be employed in other molten metal coating applications.
A typical continuous hot dip galvanizing apparatus used in zinc and zinc alloy coating of continuous steel strip is schematically shown, for example, in U.S. Pat. No. 3,499,418 which illustrates a steel strip being prepared for hot dip coating in a cleaning and annealing furnace, from which the strip passes, in a controlled atmosphere, directly into a molten zinc or zinc alloy bath. The metal strip extends downwardly into the molten metal and around a first submerged roll, referred to as a sink roll, then upwardly in contact with one or more stabilizing rolls which are also submerged in the molten metal bath. From the bath, the strip passes between a pair of opposed air knives which control the thickness of the metal coating, then upwardly through a cooling chamber to a top guide roll.
In galvanizing apparatus of the type described, the sink roll and stabilizing rolls normally are supported on arms projecting along the sides of the zinc pot into the molten metal bath. The rolls are supported by bearing assemblies each including a temperature resistant sleeve mounted on the projecting end of the roll shaft and an oversized bearing element or bushing mounted on the end of the roll support arm. Molten metal flowing into and through the oversized bushings acts as the only lubricant between the sleeve and bushing.
The temperature of the molten zinc or zinc alloy coating bath, which may be in the range of 950.degree. F., in combination with the high tensile loads required to be maintained in the strip to control its high speed movement through the apparatus, results in the roll bearing assemblies wearing quickly. With increased bearing wear, the molten zinc becomes less effective as a lubricant, thereby increasing friction which in turn accelerates wear on the bushing and sleeve.
The combination of an oversized bushing and friction load can result in roll movement, or bearing chatter, which is aggravated by bearing wear. This chatter or movement of the sink roll, and to a lesser degree of the guide rolls, can produce strip movement at the air knives and set up vibrations in the strip between the guide rolls and the top roll. Excessive movement of the strip adversely effects uniformity of coating thickness, and high frequency vibration can result in spatter of the molten coating metal and produce undesired irregularities or markings on the finished coating surface. These irregularities may adversely effect further finishing operations such as painting.
To assure against unacceptable or undesired thin spots resulting from strip movement, it has been common practice to apply an increased coating thickness as a safety margin, thereby requiring an excess of expensive coating metal. It is, accordingly, a primary object of the present invention to provide an improved hot dip coating apparatus which enables the application of a more uniform coating in a hot dip metal coating line.
Another object of the invention is to provide an improved roll bearing support system for use in a hot dip galvanizing line to provide a more stable roll position and avoid excessive wear of the bearing.
Another object of the invention is to provide such a roll support bearing system which more effectively and efficiently utilizes the molten coating metal as a lubricant to reduce wear in the bearing elements and enable a more uniform tensile load to be applied to the strip being coated.